The present invention relates to Bloch line memory devices, and in particular, to Bloch line memory devices that are suitable for use as memory apparatuses in small sized and light weight information processing equipment.
A Bloch line memory element stores information using Bloch line pairs which are microscopic magnetization structures that are generated in the magnetization structures called magnetic domain walls which are the periphery of magnetic domains that are present in magnetic films. Since the size of a Bloch line pair is about 1/10th of the width of the magnetic domain in which it is present, Bloch line pairs can offer memory densities of about an order of magnitude greater than the conventional magnetic storage devices that store information by the presence or absence of magnetic domains. If the memory density increases, it will be possible to store more information in a limited space and hence it will be possible to realize a memory device that is superior in economy and ease of handling. The basic construction and principle of operation have been described in, for example, U.S. Pat. No. 4,583,200, Japanese Patent Laid-Open No. 59-101092, etc. Further, the circuit configuration for driving such memory elements has been described in Japanese Patent Laid-Open No. 59-207011 and Japanese Patent Laid-Open No. 59-193594, etc. According to these descriptions, in order to drive a Bloch line memory element, a magnetic field generation means (coil, permanent magnet, etc.,) and the memory element are assembled in a single module which is mounted on the circuit substrate. In addition, active devices are mounted on this circuit substrate for supplying electric power to the coil. The appropriate memory operation in such a configuration was being realized by generating a magnetic field due to the electric power supplied to the coil from the active device and causing the movement of Bloch line pairs in the memory element by the magnetic field so generated.
The line shaped magnetic domains called stripe domains that constitute such a memory element are arranged by groove patterns, etc., as described in Japanese Patent Laid-Open No. 61-239487, etc. As described in this Japanese Patent Laid-Open, if a magnetic field (bias magnetic field) of more than a specific strength is applied from outside in the direction perpendicular to the magnetic garnet film, the edges of the stripe domain move in the direction to reduce the length of the stripe domain. If a groove pattern is provided within the stripe domain at this time, a repulsive force will be generated by the groove pattern on the ends of the stripe domain so that it will be possible to suppress the shrinking of the stripe domain. If the shrinking force and the repulsive force from the groove pattern can be balanced with each other due to this bias magnetic field, the length of the stripe domain can be maintained to be almost equal to the length of the groove pattern (thereby fixing the magnetic domain) even if a stray magnetic field from the outside acts on the device. Because of this function, it is possible to match the positions of the different conductor patterns and stripes required for memory operation.
In a Bloch line memory element, it is necessary to make the Bloch line pairs propagate to a specific address in order to record information and to input or output the necessary information. During propagation, a magnetic field is applied to in the direction that causes the stripe magnetic domain to shrink. At this time, it is necessary to maintain the length of the stripe magnetic domain so that is does not become shorter than necessary. In the conventional art, groove patterns were also being used for preventing this excessive shrinking of the stripe magnetic domains.
The required operation could be made by using groove patterns for fixing the stripe magnetic domains forming the Bloch line memory element thereby realizing the memory operation. Other methods have been known that have functions equivalent to this groove pattern, such as using ring shaped groove patterns described in Japanese Patent Laid-Open No. 61-248296, using magnetic garnet films with saw tooth shaped cross-section described in Japanese Patent Laid-Open No. 61-162889, or using a pattern produced by ion implantation described in Japanese Patent Laid-Open No. 62-124690, etc. In all these methods, since a fixing force is generated on the stripe magnetic domains, the required objective is achieved by balancing it with the bias magnetic field.
Further, until now, Bloch line memory devices had only the function of a simple memory device. Therefore, it was necessary to connect an external information processing equipment to process the stored information in a manner suitable for the recent trends of paperless or mediumless information distribution (as described in pages 116-136 of Nikkei Electronics Magazine of Nov. 26, 1990).
In the above described conventional art, sufficient considerations have not been given to the possibilities of attaching and detaching the Bloch line memory element with the information processing equipment in which it is used. In other words, since the Bloch line memory element and the drive circuit are formed on the same circuit substrate, it is not possible to separate out the expensive drive circuit at the time of attaching and detaching the Bloch line memory element from the information processing equipment. As a result, it is not possible to provide several circuit substrates that can be replaced due to problems of economy.
Further, in the conventional art, the Bloch line memory element is assembled in the module along with the driving coil. As a result, the thickness of the circuit substrate becomes at least about 1 cm. Consequently, it is inconvenient to be used as an external memory device in small sized and light weight equipment as compared to semiconductor (IC) cards or floppy disks, etc.
In addition, there are problems in the configuration of the memory elements also. As has already been explained above, since the stripe magnetic domains are fixed in the conventional art, it is necessary to apply a bias magnetic field to balance out the repulsive force from the fixing pattern. Therefore, it is necessary to continue to apply the bias magnetic field at all times while the information is being stored (that is, at all times other than during input and output). This is explained here in specific terms using FIG. 13a, FIG. 13b, and FIG. 13c. FIGS. 13a to 13c show an example of the method of fixing the magnetic domain using the groove patterns described in Japanese Patent Laid-Open No. 63-144487 which comprises the first groove patterns 210 that fix the magnetic domains and the second groove patterns 211 that act as a guide when the magnetic domains stretch, with these groove patterns being formed on the magnetic garnet film 220. In the condition in which the bias magnetic field has been applied as shown in FIG. 13a, the stripe magnetic domains 203 will be fixed around the first groove patterns 210. However, when the bias magnetic field is shut off as shown in FIG. 13b, the stripe magnetic domains stretch and become longer than the first groove patterns. At this time, since the ends of the stripe magnetic domains extend beyond the area containing the recording patterns (the area indicated by the arrow n), it will not be possible to transfer information. Also, if the stripe magnetic domains stretch too much, there is the danger of destroying other memory loops. Therefore, in the conventional art, the method of controlling the length of the stripe magnetic domain to be within a specific range by applying a certain amount of the bias magnetic field at all times. Further, FIG. 13c is a cross-sectional diagram that shows the state of magnetization at the cross-section B--B' in FIG. 13a.
The method of using a permanent magnet described in Japanese Patent Laid-Open No. 59-193594 is also known as a method of applying the bias magnetic field. According to this method, since a constant bias magnetic field is obtained from the permanent magnet, it will be possible to realize stable device operation. However, in this method, since it is necessary to place the permanent magnet over the memory element, the thickness of the module will increase.
Following the demands for small size and light weight of information processing equipment in the recent years, it is necessary to reduce the size and thickness of Bloch line memory devices. However, in the conventional Bloch line memory elements, since the module which is the minimum configuration for operating the element is thick, it is not possible to make the memory device think or to improve its capacity to be attached and detached easily.
Further, an external equipment having processing functions has to be connected to the conventional memory device in order to reproduce or select the information. As a consequence, several equipment will be needed to realize the functions required in paperless or mediumless information distribution systems thereby causing problems in ease of handling and portability.